I think all the bacterial stuff you were looking at were somewhere in the range of .5 to 10 microns, and the coffee filter was in the 15 micron range, so it would be like sifting flour through a wire sieve. All the stuff you were looking at in under the microscope were smaller than the pore size of your filter. All your filter did was hold back the lumps and chunks so you could see the bacteria without other debris in the way.

Sue